1. Technical Field
The technology disclosed herein relates to a light source device featuring a laser and a phosphor, and more particularly relates to a light source device that emits visible light such as red, green, blue, or other that is used in an image display device.
2. Background Information
Projectors are widely used as image display devices for projecting and enlarging various kinds of video or the like onto a screen. With a projector, light emitted from a light source is condensed or focused by a spatial light modulation element (a DMD (digital micromirror device) or a liquid crystal display element). The condensed or focused light is emitted after being modulated by an image signal at the spatial light modulation element. This emitted light is displayed on a screen as a color image.
With conventional projectors, high-pressure mercury lamps of high brightness have been used as the light source in order to obtain a bright, large-screen image. However, when a high-pressure mercury lamp is used as the light source, there is an environmental drawback in that the lamp contains mercury, which is a hazardous substance. Another problem with high-pressure mercury lamps is that they have a short lifetime as a light source, and they are difficult to maintain.
To solve these problems, there have been proposals for light source devices that make use of solid-state light source such as a light emitting diode (LED), laser, or other instead of a high-pressure mercury lamp, as well as for image display devices in which these light source devices are used.
A laser light source has a longer lifetime than a high-pressure mercury lamp. Also, a laser light source produces coherent light, so the light has good directionality and is utilized more efficiently. Furthermore, thanks to its monochromaticity, a laser light source can reproduce colors over a wide range.
Nevertheless, because of its high interference, a problem with laser light is that speckle noise is produced, which adversely affects image quality. Furthermore, when laser light is emitted at high output, physical safety must be ensured to avoid accidental irradiation of the human eye by the high-output laser beam.
Also, an LED light source is less prone to the above-mentioned problems of speckle noise and safety issues. However, with an LED light source, the emission surface area of the light source is large, and the optical energy density of a green LED is particularly low. Accordingly, with an LED light source, at present it is difficult to obtain a high-brightness image display device using LED's of three colors (red, green, and blue).
Meanwhile, light source devices and illumination devices have been proposed that make use of an LED light source or laser light source as an excitation source, and light emitted from a phosphor as a solid-state light source. Image display devices equipped with the above-mentioned devices have also been proposed.
With a light source device involving a phosphor, a high-brightness light source with a small emission surface area can be obtained by using a laser or the like capable of high-density convergence as the excitation light source. Also, even when a laser is used as the excitation light source, the fluorescent light obtained by frequency conversion is incoherent light. Accordingly, with this type of light source device, no speckle noise is generated and no convergence of laser light occurs, so such devices are favorable in terms of physical safety.
To obtain a highly efficient light source device that can be applied to an image display device, it is preferable for the emission surface area of the light source component to be as small as possible. However, if the spot diameter of the laser beam condensed or focused on the phosphor is made too small, this will result in a problem in that the temperature of the phosphor increases and the emission efficiency of the fluorescent light decreases. This is called temperature quenching, and is a serious technological problem when using a phosphor.
With an illumination device for an image display device, it is generally necessary to use illumination light having a rectangular spatial intensity distribution that matches the shape of the DMD or liquid crystal display-equivalent element. Therefore, illuminance homogenizer such as a rod integrator or other having a rectangular cross section is sometimes used. In this case, to obtain an efficient light source device in which a phosphor is used as the light source, it is effective for the shape of the emission face of the phosphor to be substantially equivalent to the shape of the rod integrator.
In Japanese Laid-Open Patent Application 2011-13320 (Patent Literature 1), a blue semiconductor laser is used as the excitation light source, and a reflective configuration is used in which the phosphor is disposed on a rotating reflective substrate to take off light to the rear. In Japanese Laid-Open Patent Application 2010-181529 (Patent Literature 2), a discharge lamp that generates ultraviolet light is used as the excitation light source. In Patent Literature 2, a transmissive configuration is used in which the phosphor is disposed on a rotating transparent substrate to take off light to the front.
In Patent Literature 1, a blue semiconductor laser is used as the excitation light source, and a reflective configuration is used in which the phosphor is disposed on a rotating reflective substrate to take off light to the rear. In Patent Literature 1, the obtained fluorescent light is used as green illumination light for an image display device, but since the spatial distribution of the laser beam spot on the phosphor is not strictly controlled, problems are that (1) the spatial intensity distribution shape of the fluorescent light on the phosphor does not match the incident end face shape of the rod integrator, so optical loss occurs, and (2) the laser beam spot on the phosphor is too small, so the effect of temperature quenching is considerable.
In Patent Literature 2, a discharge lamp that generates ultraviolet light is used as the excitation light source, and a transmissive configuration is used in which the phosphor is disposed on a rotating transparent substrate to take off light to the front. In Patent Literature 2, a rod integrator is inserted between the excitation light source and the phosphor, and the spatial intensity distribution of the excitation light directed at the phosphor is matched to the same rectangular shape as the DMD, but since a gap is required between the rod integrator and a phosphor module having a rotary mechanism, the excitation light spot on the phosphor ends up spreading out considerably, so it is difficult to obtain a highly efficient illumination device.
The present technology is proposed in light of the above situation, and provides a light source device that uses a phosphor to obtain illumination light of high brightness and high efficiency.